Cathode-ray tube deflection system



May 30, 1959 c. E. ToRscH 2,510,027

cA'rHoDE-RAY TUBE DFLECTION SYSTEM Filed May 2a, 1947 n HomzoNTAL 7PAnAaoLA FoRMlNG sPHERlcAL sunFAcE 'IDFA or RADlus=BAM THRow g FROMAPPARENT CENTER 51'24 or MAGNETIC DEFLECTION SAWTOOTH FORMING "FIGAI /oA 5 B lo G lsnzogzspaogssww; 4sJsoKss so es mo'r) o /TRACE ns1-RACESECTOR INVENTOR GW m ENSDCNJJ CHARLES ETORSCH ATTORN EY 55 mlcrosecondsBY Patented May 30, 1950 UNITED STATES @ENCE CATHODE-RAY TUBE DEFLECTIONSYSTEM Charles E. Torsch, Towson, Md., assignor to Radio Corporation ofAmerica, a corporation of Dela- Application May 28, 1947, Serial No.750,934

Y 11 Claims. 1

This invention relates to television, and more particularly to improvedcathode ray beam deflection signal generators.

Image reproduction by electronic viewing devices has been generallyaccepted as the most desirable and has met with outstanding success.Consequently, the cathode ray viewing tube is almost universally usedtoday in television receivers.

The reproduction of images is accomplished by analyzing the scene intoits picture elements by a predetermined sequence of scanning andtransmitting energy representative of each picture element forreconstruction at the receiving point by the same scanning sequence.

In order to produce scanning, the beam must be moved across the screenindependently in two directions and at right angles toeach other. It ispreferable to deflect the beam through an angle at a position close tothe end of the gun. This deection of the electron beam can beaccomplished by either an electrostatic field produced by a pair ofdeecting plates or a magnetic field set up at right anglesto vthedirection oi deflection and produced by an inductance coil. Each ofthese methods has been successfulV in practical application as a meansfor producingrelectron beam deflection. V

Magnetic deflection for both vertical and horizontal displacement of theelectron beam has become almost universal in application for employmenton large cathode ray tubes.

Magnetic deflection is accomplished by estab- If the tube is providedwith suitable shaped coils and cores, a perfect scanning pattern can beobtained only if they are supplied with current 01 voltage having a veryspecific time variation. The wave form of current through the inductancecoils producing the vertical deflection of the electron beam must besawtoothed andrhave a recurring frequency equal to the neld frequency ofthe pattern. The current through the horizontal coils must also besawtoothed, but its repetition frequency is much higher and correspondsto the desired line frequency. The current in each case builds upsubstantially linearly with time for most of the cycle, and then returnsrapidly to its initial value.

The deflection signal generator which supplies the power to thesedeflecting elements generally consists of an oscillator, a sawtoothgenerator and a deflection amplifier. The oscillator is normally set offby a series of synchronizing pulses which serve to coordinate the timingof the scanning rasters at the transmitter and receiver.

Although the early forms of cathode ray deflection systems generallyemployed a simple gaseous discharge sawtooth Wave generator, the exactrequirements of television in its improved forms have placed stringentrequirements for both speed and velocity on the deflection system tomake it necessary to employ thermionic devices together with waveshaping circuits to furnish the necessary deiiection energy.

There are three distinct portions of the line deecting cycle, which maybe outlined as (a) the major portion of the go sweep, during which timethe output tube of the deflection generator supplies current to thedeflection yoke, (b) the interval of free oscillation (retrace time)during which time the output tube plate current is cut off and the waveform is determined by the circuit constants of inductance, capacity, andresistance, and (c) the portion following the return sweep, during whichthe free oscillation must be damped and energy which has been stored inthe yoke inductance is permitted to be discharged, preferably in acontrolled manner.

The driver tube portion of the go cycle was of major interest in earlydesigns of cathode ray tube systems. Particular emphasis was placed uponobtaining a yoke current, which was a linear function of time.

However, the cathode ray tubes in general use today for televisonVsystems and the like are of the fiat-faced wide-angle kinescope types.VVIt therefore becomes necessary to modify the yoke current to allow forthe deflection geometry, ofY such tubes. For a properly designeddeflection yoke, the deflection angle is approximately a linear functionof the yoke current. Therefore, for small deflection angles,particularly when the tube face radius approximates the deflectionradius, the spot, displacement is a linear function of the yoke current.However, when the tube face radius and the deflection radius differradically, and particularly for Wide deflection angles, the yoke currentmust be distinctly non-linear in order to obtain a linear spotdisplacement.

According to this invention, there is provided an improved deflectionsystem wherein the deflection current is conveniently adjustable toprovide the necessary non-linearity in change of cur- 3 rent to obtainthe necessary linear scanning spot displacement.

The interval of free oscillation or retrace time during which the outputtube plate currentV is cut off has heretofore been taken care of byutilizing a high degree of damping. Part of this damping was in somecases inadvertently slighted by the output tube which, althoughsupposedly at cut-ofi insofar as grid signal Was concerned, frequentlywas not cut oil. Additional damping was introduced by losses in thecoupling transformer and yoke, and in some cases sufficient additionalresistance Was added in shunt with the yoke in order to make the circuitaperiodic. this method of damping had the disadvantage of causing lossesduring the go sweep, as Well as during the return sweep, and hence thedeection efficiency of the system was low.

It has been proposed to improve the circuit characteristics in efciencyby using a series resistancecapacity combination for damping. Thistended to decouple the damping resistor during the slower go time andyet provide close coupling to the damping load during the rapid returntime. Generally satisfactory results were obtained for the relativelyslow 441-line standards, however, with the introduction of the 525- linestandard, the return time of an aperiodic system is rather lo-ng, sothat new damping procedures were proposed. Y

Circuits employing triode and diode damping in the secondary of thedeection transformer have met with considerable success, however, aperfect coupling in a deection transformer is difcult to obtain andwould be costly to produce, due primarily to the high voltage insulationwhich is required.Y A certain amount of decoupling with the dampingelement across the secondary of the de-Y flection transformer results ina carry-over voltage in the primary of the deection transformer whichcauses the output tube to produce spurious oscillation, such as theWell-known Barkhausen oscillations.

The cause of the Barkhausen eiects can be attributed to the negativeexcursion of the anode potential of the output tube due tothe'transients obtained upon a rapid change in current through aninductive element such as the deection transformer.

According to this invention, there is provided a' other and incidentalobjects of the invention' will be apparent to those skilled in the artfrom a reading of the following specification vand an inspection of theaccompanying drawing i which:

Obviously, Y

Y circuit comprising the capacitive element 3 and Figure 1 illustratesschematically one formfof Y a resistance element 5.

The generation of sawtooth Wave forms in accordance with a predeterminedsynchronizing pulse is wellnown in the art and needs no detailedexplanation here, except perhaps to refer to Chapter IV, beginning onpage 118 of Principles of Television Engineering by Donald G. Fink,wherein the formation, deflection and synchronization of scanning beamsis very Well described in detail. Deilecting circuits are also wellknown in the patented art, such as, for example, the U. S; patent to W.A. Tolson et al. No. 2,101,520, dated ,December 7, 1937; Vand the U. S.patents to W. A. Toison Nos. 2,108,152, dated February 15,

1938, and 2,280,733, dated AprilV 21, 1942.

The horizontal or line deilecting yoke 1 of the cathode ray tube siscoupled to the output of tube I by transformer I I.

Image producing tubes of the kinescope type are also well known -to theart, and a description may be found'in an article byDr. V. Zworykin"entitled Description of an experimental teievision system and kinescope,in the Proceedings of the Institute of Radio Engineersfvol. 21, No. 12,for December 1933, or the information begin ning on page 329 of the bookTelevision by V. K.

Zworykin and G. A. Morton, published in 1940-by vention is, of course,applicable to` any scanning.

device.

A typical deflection yoke is shown and'described in detail in the U. S.patent to W. A. Tolson, No. 2,167,379, dated July 25, 1939.

It is well established that the' magnetic iield set up by the deectionyoke 1 is proportional to the product of the number of turns in the coiland the current flowing in' them. Therefore, the amount of currentrequired in the coils dependsv directly on the number of turns in thecoils. It

-is important, particularly in circuits involving `rapid deflection,that not too large a value of inductance be employed. VIt is thereforenecessary to supply a relatively large value of current to thedeiiecting yoke '1. It follows that lhigh currents will necessitate theconsideration rin the deilectionv system of extremely high peakedvoltages which are `generated in the inductance coils when the-V currentthrough them changes suddenly.

The scanning yoke has associated with. it a distributedcanacitancewhich, acting with the in-v ductance of the yoke, forms a resonantcircuit. The sudden shck'caused by the sawtooth wave form of thedriving'current tends tov set the reso-` nant circuit into oscillation.The undesired oscillations in the voltage wave form producevcorresponding irregularities in the current wave K form which arereproduced in the scanning motion.

It has been Athe general practice to employ al rectifier, such as thediode damping tube i3 or any other form of runit suchv .as one ofthesemiconductor crystals, to reduce the undesired oscillation. The' diodeI3 is connected across the circuit involving the secondary of thetransformer II and the deilecting yoke 1 and is connected in series witha capacity I5 and resistance element I1.. Resistance element I1 may bemade variable as shown.

The operation of the damping tube I3 is well knownin the art, andadditional information thereon may be obtained by reference to acopending U. S. application of Otto H. Schade, Serial No. 593,161, filedMay 11, 1945, wherein there is shown and described a damping tube,together with power feed-back for utilization of the rectified currentsin conjunction with the output tube anode potential.

If the transformer II were so constructed that it would have perfectcoupling, the effect of the diode damper, tube I3 would be projectedback to the primary of transformer II in such a manner that there wouldbe no negative excursion of voltage on the plate of tube I due to theextremely high peak voltages employed. However, in the practicalconsideration and design of transformer II, it is necessary to arrive ata compromise on coupling such that the Veffect of the inductance on thetransformer is to cause a negative excursion of Voltage on the tube I.

The negative excursion referred to may best be understood by referenceto Figure 3, which for the most part is believed to be self-explanatory,except perhaps to state that the wave shape shows the plate voltage of atypical power output tube such as a 6BG6--G, which oscillates to 1900volts at the beginning of the forward trace for go sweep. This negativeexcursion will cause the output tube to produce Barkhausen oscillations,which may readily affect the radio frequency section of the televisionreceiver at high frequencies. The oscillations will be carried throughthe circuit to modulate the kinescope grid and will appear as verticalbars at the left edge of the image. The effect of the Barkhausenoscillations will not be observed whenthe scanning circuit alone istested. The `effect is generally found .only when the complete receiveris in operation and tuned to the higher frequency channels and when highintermediate frequency gain is employed.

Returning now to Figure l, there is included a tube I9 having itscathode 2| connected to an auxiliary control electrode 23 of tube I. Theanode of tube I9 is connected to a source of positive potential. Thecontrol electrode 26 of tube i9 is connected to the anode of diode I3through a wave forming circuit comprising, for example, a pair ofpotentiometers 25 and 21. Potentiometer 25 is adapted to supply avariable current to a parabolic wave forming network including resistors29 and condensers 3l. Potentiometer 21 is adapted to supply a sawtoothwave forming circuit including resistors 33 and condenser 35. 'Iheoutput signal from the parabolic wave forming circuit and the sawtoothwave forming circuit is fed to the control electrode 24 through capacity31. A D.C. blocking condenser 38 is included in the circuit betweendiode I3 and potentiometers 25 and 21.

The function of tube I9 is to drop the potential of auxiliary controlelectrode 23`of tube I 'during the negative excursion of plate voltageof tube I. A suitable delay of the negative surge on the secondary ofelement II through adjustment of 33 and 20, and amplitude adjustmentwith 25 and 21 during this interval will cause control electrode 24 togonegative. This in effect opens the circuit at tube I9 so that thepotential of control electrode 23 of tube I will go immediately towardsor below its associated cathode potential. By dropping the potential ofcontrol electrode 23 to a sufficiently low value, there will beprevented in tube I any tendency toward Barkhausen oscillations.

It may also be added that the useless negative excursion referred tounnecessarily abuses the output tube I insofar as interelectrodeinsulation stresses are concerned and furthermore, the power obtainablefrom the output tube I is normally limited by the screen dissipation.The employment of this invention, therefore, reduces the abuses of theoutput tube, but permits a greater eiciency of operation. It is alsopossible to employ lower plate voltages in the output tube by obtaininggreater eiliciency in its operation.

In addition to preventing Barkhausen oscillations, the tube I9 may beutilized to control the wave shape of the deflection voltages throughthe parabola forming and sawtooth forming networks including elements20, 3|, 33 and 35. By properly selecting the values of these elements,many desired wave shapes may be obtained. Other wave shapes may beobtained by returning 25 and 21 to an extension of the secondari7 of IIbelow the tap herein connected to +B, instead of grounding 25 and 21.This extension would generate positive voltage surges during retrace andpermit the generation of opposite polarities of parabola and sawtoothwaves.

The adjustable wave shapes of the deflection current may be used toadvantage, for exampie, in providing the non-linear yoke currentrequired to obtain a linear spot displacement in the modern kinescopetubes having a substantially flat tube face. This will be more readilyunderstood after a brief reference to Figures 2 and 4.

In Figure 2, there is shown a graphic determination of required yokecurrent wave shape on the basis of face-plate curvature. The curva tureas illustrated in Figure 2 is typical of commercially producedkinescopes and is taken by way of example from the '1DP4 and 10BP4types.

In Figure 4 there is shown a yoke current curve. The dashed line isstraight and would be desirable for a tube having a spherical screensurface whose radius is equal to the beam throw from the apparent centerof deflection. When, however, the surface is flattened out, asillustrated in Figure 2, there is less deflection rate required at thebeginning and end of the course, hence it is necessary to provide a yokecurrent as shown in the solid line of the curve shown in Figure 4. Sucha yoke current curve, as well as other forms, may be provided by the useof this invention.

Linearity measurements usually refer to meas urements made at the faceof the kinescope. The unit length commonly used as the basis ofproduction linearity testing is 10% of the useful scan. If the number ofbars per unit length is within i10% of the number scanned at the centerof the screen, then the linearity is said to be 10%. Strictly speaking,the spot velocity is what is actually being measured if the intervals ofmeasurement are sufficiently small. Methods forfmeasurement ofthe'angular velocity of the deection beam may be coordinated with thevelocity data of Figure 2 to provide an approximate method ofmeasurement. In this event, the free-running speed of the blockingoscillator should be adjusted to .the nominal horizontal line frequency.

Having thus described .the invention, what .is claimed is:

1. In a beam deflection system, ibeam deflection means, a power tube.having fan output cir cuit and an `auxiliary control electrode, ,acoupling circuit between said output circuit and .said beam deflectionmeans, va control voltage circuit connection between said beamdeflection :means and said auxiliary control electrode, said controlvoltage circuit connection Y.including parabola Wave forming network.

2. In a television deflection system, a-beam ideflection coil, a powertube having an voutput circuit and an auxiliary control electrode, acoupling .circuit between said output circuit andsaid beam deilectioncoil, and means for reducing spurious oscillations in said beam deectioncoil comprising a control voltage circuit connection between said beamdeilection coil Vand v`said auxiliary control electrode, saidcontrolvoltage circuit connection including a tube having a cathode connectedto said auxiliary control Velectrode and a control electrode connectedto said beam deflection coil through a wave form changing network.

3. In a television deilection system, beam deflection means, a powertube having -an output circuit and an auxiliary vcontrolelectrode, -acoupling circuit between said output circuit and said beam deflectionmeans, and a control voltage ci-rcuit connection containing a cathodefollower type tube connected between said beam deflection means and saidauxiliary control electrode to control the potential of said auxiliarycontrol electrode during predetermined time intervals.

4. In a television deflection system, beam deflection means, a powertube having an output circuit and an auxiliary control electrode, acoupling circuit between said output circuit and .said beam deflectionmeans, and 4-a cathode follower connected amplilier tube connectedbetween Vsaid. beam deflection means and said auxiliar-y controlelectrode to controlthe potential of said auxiliary control electrodeduring voltage surges .across said beam deflection means.

5. In a television deflection system, beam deilection means, a powertube having an output circuit and an auxiliary control electrode, vacoupling circuit between said output circuit and said beam deflectionmeans, and a wave shaping vcircuit connection between .said beam.deflection means and said auxiliary .control electrode, said waveshaping circuit comprising a cathode -coupled amplifier tube connectedserially vwith `a parallel connected sawtooth wave and parabola waveforming electrical networks.

y6. In a television deflection system, beam deflection means, a powertube having an input circuit adapted to receive a sawtooth wave, anoutput circuit and a current control electrode, a coupling circuitbetween said output circuit land said beam deflection means, and aparabola wave forming network connected between said beam deflectionmeans and said current control electrode.

7. In a television deflection system, beam deflection means, a powertubehavingan input circuit adapted to receive a sawtooth wave, an outputcircuit and a current control electrode, a Vcou-- pllngcircuitrbetweensaid loutput circuit andsaid beam 'deflection means, yandalsawtooth wave and parabola. wave. forming network connected between.said beam deflection means fand-,said current control electrode.

8. In ya television deflection system, beam de# flection means, a powertube having an input circuit adaptedto receive Ia sawtooth wave, vanoutput circuit and a current control electrode, a `coupling. circuitbetween said outputcircuitand said beam deflection means, an auxiliarycontrol tube having a cathode, a control .electrode and an anode, saidcathode Vconnected to said current control electrode, .said .anodeconnected to .a source of positive potential, and .a control voltageconnection between said beam deflection means and said controlelectrode.

9. Ina television deflection system, beam deflection means, a power tubehaving an input eircuit adapted to receive a vsawtooth wave, an outputcircuit anda current controlelectrode, acoupling circuit between ,saidoutput circuit and said beam deflection means, an vauxiliary controltube have a cathode, a control electrode and an anode, said cathodeconnected 'to said .current control electrode, said anode connected to asource of positive potential, and a circuit connection between said beamdeflection means .and .said control `electrode including serially'connected resistances and parallel connected capacity,

110. In a television deflection system, beam deilection means,a-'powertube .having an input circuit adapted Yto receive a sawtoothwave, an :output circuit and a current control electrode, a couplingcircuit `betweensaid output circuit and said beam deflectionv means, an`auxiliary control tube having a cathode, a .control electrode and ananode, said cathode connected to said current control electrode, saidanode connected to a source of positive potential, and a circuitconnection between said beam deflection means and said control electrodehaving serially lconnected resistance and parallel .connected capacity.

1'1. In a television deflection system, beam deflection means, -a powertube'having an input circuit adapted'to receive asawtooth wave, Ianoutput circuit and a'current control electrode, .a coupling Vcircuitbetween said output circuit and said beam deflection means, an auxiliarycontrol tube having a cathode, a control electrode Vand an anode, saidcathode connected to `said current control electrode, said anodeconnected to a source of positivepotential, and a sawtooth wave andparabola wave forming networks connected between said beam deflectionmeans and `said control electrode.

' CHARLES'E. TORSCIL REFERENCES CITED The following references Vare ofrecord in the le of this patent?" l UNITED z's'rA'rfEs PATENTS

